The trend in the magnetic recording art is toward higher capacity, higher transfer rate, faster access, and increased system bandwidth through the recording and playback of shorter and shorter wavelength signals. This trend necessitates intimate contact at the interface between the magnetic head and the recording medium.
In order to obtain optimum performance in writing to and reading from magnetic tape, it is necessary that the moving tape be kept in close and stable proximity to the magnetic head. As the head-to-tape separation or flying height increases, performance deteriorates. The primary consequences of a higher tape flying height are a decrease in read amplitude and an upward shift in the peak write current. Conversely, the read amplitude increases as the flying height decreases. Therefore, an increase in tape head performance can be obtained by minimizing flying height.
The magnetic head contour is typically cylindrical. At higher tape speeds, an air layer adherent to the moving tape becomes entrained between the magnetic head and the tape surface traversing the head, causing the magnetic tape to "fly" over the head. The entrained air acts as an air bearing, lifting the tape from the head contour and separating the recording medium from the interface, with resultant signal degradation.
Intimate contact between the tape and the magnetic head at the interface is typically increased by a combination of greater tape tension across the magnetic head and more penetration of the magnetic head into the tape. Another approach to minimizing flying height on cylindrical magnetic heads is to incorporate bleed slots. Bleed slots are grooves in the contour surface. As the tape moves across the head, the bleed slots help to channel entrained air away from the head-to-tape interface, thus reducing the height distribution of the layer of air. Thus, bleed slots function in a manner that is analogous to treads on a tire. Just as the tire treads help to channel water away from the tire surface to prevent hydroplaning, bleed slots help to channel away air from the head contour surface to minimize head-to-tape separation.
An optimized head contour design is typically based on a cylindrical surface having a radius of curvature of about 4 to about 8 millimeters and air bleed grooves, either longitudinally or transversally, to maintain minimal head-to-tape spacing. In a typical head-to-tape interface, head-to-tape penetration of about 3 to about 5 millimeters in combination with about 80 to about 120 Newtons per meter of tape tension to create adequate downward force to achieve close head-to-tape spacing. However, these conditions can lead to increased head wear and tape damage. Excessive wear may result in either alterations of the head profile or erosion of the gap in the case of thin film heads.
For example, after prolonged usage, an undulating wear profile is typically created on the magnetic head in a direction perpendicular to the motion of the magnetic tape. This profile is due to the non-uniformity in the head-to-tape separation across the tape. In other words, the flying height of the tape varies from point to point across the tape. At points of low flying height, wear of the head is more pronounced than at points of greater flying height.
FIG. 1A is a schematic illustration of a plain flat magnetic head 20 with the read-write elements 22 located in a flat region 24, as generally set forth in Hinteregger et al., Contact Tape Recording with a Flat Head Contour, IEEE Transactions on Magnetics, Vol. 32, No. 5, Pg. 3476, September 1996. The configuration of FIG. 1A generates a region of sub-ambient pressure at the interface 26 of the tape 28 moving in the direction 29 and the flat surface 24. FIG. 1B is a graphical illustration of the pressure at the interface 26 across the length of the magnetic head 20. Air is entrained by the tape 28 at the leading edge 30 of the magnetic head 20, such that the head-to-tape spacing has a slightly diverging shape 32. Consequently, the air pressure in the region 32 is sub-ambient, as illustrated in FIG. 1B. The air expands in the region 32 between the tape 28 and the flat region 24 and creates suction under the tape 28. The air pressure remains generally constant and sub-ambient and the head-to-tape spacing is remains generally constant across the remainder of the flat region 24.
The highest contact pressure occurs at the leading edge 30 of the magnetic head 20. With low wrap angles, typically in the range of about 2.degree. and reduced tape tension, the center region 34 of the tape 28 can be subject to instability due to tape flutter. This instability is sensitive to variations in tape speed and tape tension. Instability in the tape 28 in the center region 34 can increase wear on the read-write elements 22 and degrade electrical performance of the magnetic head 20.